In a conventional technology, there has been provided a problem to suppress flow-accelerated corrosion such as an elution of ions from and corrosion in a metal that is a structural material, in various plants such as a thermal power plant, a nuclear power plant and a chemical plant, which have a boiler, a steam generator and the like, and which use a high-temperature water of 80° C. or higher and 800° C. or lower. The elution of metal ions is a representative phenomenon occurring in the high-temperature water, which provides problems, for example, of causing various influences in operation, and increasing in frequency of maintenance of the plant due to the corrosion of a pipe and other members including the structural material.
In addition, the eluted metal ion adheres to and deposits on a surface of the pipe in a system, or a high temperature site or a site with which high-temperature water comes in contact, in the steam generator and the like, as an oxide, which may result in a possibility of causing a vibration of equipment and the lowering of a heat transfer coefficient. The vibration under a high flow-velocity condition is a serious problem, and has a possibility of causing a crack or damage in the structural material. Moreover, a recent technology may cause a possibility of decreasing in the thickness of a carbon steel pipe due to a wall-thinning phenomenon in the pipe and a malfunction occurs.
As mentioned above, there is a potential possibility that the elution and corrosion phenomena and the like of the metal are gradually accumulated during a plant operation in a long period of time and a malfunction suddenly occurs at some stage. Furthermore, there is a possibility that a corrosion rate increases according to shape of a structural portion or an unpredictable phenomenon occurs.
For instance, in a piping system in which orifices, valves and the like are much used, there is a possibility of causing erosion-corrosion by a liquid which passes through the inner space at a high speed. On the other hand, in a narrow portion such as a crevice portion between a heat transfer pipe and a supporting plate, there is a possibility of accumulating impurities at a highly concentrated state, and occasionally forming concentrated water having strong acidity or strong alkalinity according to the ion balance. A phenomenon of corrosion cracking is confirmed, which is caused by such a phenomenon and a rise of an electric potential, due to the oxide adhering to the surface. Because of these reasons, various measures including a water quality control have been implemented in various plant systems.
For instance, in the secondary system of the pressurized-water nuclear power plant, such measures are taken as to control pH by injecting ammonia and hydrazine, to decrease the elution of iron from the inside of the system and to prevent the inflow of the iron to the steam generator (Patent Document 1, for example, described hereinbelow).
Furthermore, in order to eliminate alkali concentration in the crevice portion, various water quality controls have been implemented in an actual plant, such as an Na/Cl ratio control, a chloride ion concentration control for decreasing an influence of a chlorine ion on corrosion, and a dissolved oxygen concentration control (Patent Document 2, for example). In recent years, a water-quality control method is also adopted by using improved chemicals such as ethanolamine and morpholine.
In addition to the measures already implemented in an actual plant, such as reductions of the corrosion of pipes, the adhesion and deposition of an oxide, and the concentration of ions in the crevice portion as described above, various technologies have also been proposed as an improved proposal. For example, for the improvement of the chemicals to be injected, there has been proposed a method of using an organic acid such as tannic acid and ascorbic acid as an oxygen scavenger (Patent Document 3).
Furthermore, as for the water-quality control method, there are also proposed an operation method of controlling a molar ratio of all cations/SO4 (Patent Document 4), a method of introducing at least one of a calcium compound and a magnesium compound into feed-water to a steam generator for a reactor so that the ion concentration becomes 0.4 to 0.8 ppb (Patent Document 5).
As mentioned above, the measures of suppressing corrosion elution by a water-quality control technique and the like which uses the chemicals are actually implemented as a measure of preventing the corrosion and elution of a plant structural material. However, it is needed and desired to enhance an availability factor and a reliability of the plant by providing a member of a plant which can be operated without injection of the chemical liquid, from the viewpoint of operation cost and operability,